1. Technical Field
The present disclosure relates to a system and method for measuring the specific absorption rate of electromagnetic energy emitted by energy-delivery devices, such as energy-emitting probes or electrodes, and, more particularly, to specific absorption rate measurement and characterization of energy-delivery devices using image analysis.
2. Discussion of Related Art
Treatment of certain diseases requires the destruction of malignant tissue growths, e.g., tumors. Electromagnetic radiation can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the probes are positioned, electromagnetic energy is passed through the probes into surrounding tissue.
In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic radiation to heat, ablate and/or coagulate tissue. Microwave energy is sometimes utilized to perform these methods. Other procedures utilizing electromagnetic radiation to heat tissue also include coagulation, cutting and/or ablation of tissue. Many procedures and types of devices utilizing electromagnetic radiation to heat tissue have been developed.
In treatment methods utilizing electromagnetic radiation, such as hyperthermia therapy, the transference or dispersion of heat generally may occur by mechanisms of radiation, conduction, and convection. Biological effects that result from heating of tissue by electromagnetic energy are often referred to as “thermal” effects. “Thermal radiation” and “radiative heat transfer” are two terms used to describe the transfer of energy in the form of electromagnetic waves (e.g., as predicted by electromagnetic wave theory) or photons (e.g., as predicted by quantum mechanics). In the context of heat transfer, the term “conduction” generally refers to the transfer of energy from more energetic to less energetic particles of substances due to interactions between the particles. The term “convection” generally refers to the energy transfer between a solid surface and an adjacent moving fluid. Convection heat transfer may be a combination of diffusion or molecular motion within the fluid and the bulk or macroscopic motion of the fluid.
The extent of tissue heating may depend on several factors including the rate at which energy is absorbed by, or dissipated in, the tissue under treatment. The electromagnetic-energy absorption rate in biological tissue may be quantified by the specific absorption rate (SAR), a measure of the energy per unit mass absorbed by tissue and is usually expressed in units of watts per kilogram (W/kg). For SAR evaluation, a simulated biological tissue or “phantom” having physical properties, e.g., dielectric constant, similar to that of the human body is generally used.
One method to determine the SAR is to measure the rate of temperature rise in tissue as a function of the specific heat capacity (often shortened to “specific heat”) of the tissue. This method requires knowledge of the specific heat of the tissue. A second method is to determine the SAR by measuring the electric field strength in tissue. This method requires knowledge of the conductivity and density values of the tissue.
The relationship between radiation and SAR may be expressed as
                                          S            ⁢                                                  ⁢            A            ⁢                                                  ⁢            R                    =                                    1              2                        ⁢                          σ              ρ                        ⁢                                                          E                                            2                                      ,                            (        1        )            where σ is the tissue electrical conductivity in units of Siemens per meter (S/m), ρ is the tissue density in units of kilograms per cubic meter (kg/m3), and |E| is the magnitude of the local electric field in units of volts per meter (V/m).
The relationship between the initial temperature rise ΔT (° C.) in tissue and the specific absorption rate may be expressed as
                                          Δ            ⁢                                                  ⁢            T                    =                                    1              C                        ⁢            S            ⁢                                                  ⁢            A            ⁢                                                  ⁢            R            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                          ,                            (        2        )            where c is the specific heat of the tissue (or phantom material) in units of Joules/kg-° C., and Δt is the time period of exposure in seconds. Substituting equation (1) into equation (2) yields a relation between the induced temperature rise in tissue and the applied electric field as
                              Δ          ⁢                                          ⁢          T                =                              1            2                    ⁢                      σ                          ρ              ⁢                                                          ⁢              c                                ⁢                                                  E                                      2                    ⁢          Δ          ⁢                                          ⁢                      t            .                                              (        3        )            
As can be seen from the above equations, modifying the local electric-field amplitude directly affects the local energy absorption and induced temperature rise in tissue. In treatment methods such as hyperthermia therapy, it would be desirable to deposit an electric field of sufficient magnitude to heat malignant tissue to temperatures above 41° C. while limiting the SAR magnitude in nearby healthy tissue to be less than that within the tumor to keep the healthy cells below the temperature causing cell death.
SAR measurement and the characterization of energy-delivery devices may ensure clinical safety and performance of the energy-delivery devices. SAR measurement and characterization of energy-delivery devices may generate data to facilitate planning and effective execution of therapeutic hyperthermic treatments.